As a refrigerant can produce the cooling effect.

Suppose a vessel of ammonia (boiling point at atmospheric pressure equal to -33 ° C) inside an insulated box and with a suction tube provided with a valve, discharging into the atmosphere. At 30 ° C, the pressure inside the vessel will be 10.86 kg / cm² relative (or 154.5 psig) and nothing will happen. By opening the suction valve, there will be steam escaping from ammonia, reducing the pressure. As the pressure decreases, the boiling point also decreases, and the liquid will continue to boil as long as there is sufficient heat in the liquid or contents of the insulated box. If the suction valve is left open until the pressure is equal to atmospheric, the ammonia temperature will be reduced to -33 ° C. Then the inside of the box will be near this temperature, when the evaporation of the ammonia will cease. Any added heat will cause the ammonia to boil again and the heat removed